This invention relates to novel tin-containing compositions and their use.
As the uncertain nature of ready supplies and access to crude oil has become increasingly apparent, alternative sources of hydrocarbons and fuel have been sought out and explored. The conversion of low molecular weight alkanes (lower alkanes) to higher molecular weight hydrocarbons has received increasing consideration as such low molecular weight alkanes are generally available from readily secured and reliable sources. Natural gas, partially as a result of its comparative abundance, has received a large measure of the attention focused on sources of low molecular weight alkanes. Large deposits of natural gas, and mainly composed of methane, are found in many locations throughout the world. In addition, low molecular weight alkanes are generally present in coal deposits and may be formed during numerous mining operations, in various petroleum processes, and in the above- or below-ground gasification or liquefaction of synthetic fuelstocks, such as coal, tar sands, oil shale and biomass, for example. In addition, in the search for petroleum, large amounts of natural gas are discovered in remote areas where there is no local market for its use as a fuel or otherwise. Additional major natural gas resources are prevalent in many remote portions of the world such as remote areas of western Canada, Australia, U.S.S.R. and Asia. Commonly, natural gas from these types of resources is referred to as "remote gas".
Generally much of the readily accessible natural gas is used in local markets as the natural gas has a high value use as a fuel whether in residential, commercial or industrial applications. Accessibility, however, is a major obstacle to the effective and extensive use of remote gas. In fact, vast quantities of natural gas are often flared, particularly in remote areas from where its transport in gaseous form is practically impossible.
Conversion of natural gas to liquid products is a promising solution to the problem of transporting low molecular weight hydrocarbons from remote areas and constitutes a special challenge to the petrochemical and energy industries. The dominant technology now employed for utilizing remote natural gas involves its conversion to synthesis gas, also commonly referred to as "syngas", a mixture of hydrogen and carbon monoxide, with the syngas subsequently being converted to liquid products. While syngas processing provides a means for converting natural gas to a more easily transportable liquid that in turn can be converted to useful products, the step of forming synthesis gas as an intermediate is typically relatively costly as it involves adding oxygen to the rather inert methane molecule to form a mixture of hydrogen and carbon monoxide. While oxygen addition to the carbon and hydrogen of methane molecules may be advantageous when the desired products are themselves oxygen containing, such as methanol or acetic acid, for example, such oxygen addition is generally undesirable when hydrocarbons such as gasoline or diesel fuel are the desired products as the added oxygen must subsequently be removed. Such addition and removal of oxygen naturally tends to increase the cost associated with such processing.
Methane, the predominant component of natural gas, although difficult to activate can be reacted with oxygen or oxygen-containing compounds such as water or carbon dioxide to produce synthesis gas. Synthesis gas can be converted to syncrude such as with Fischer-Tropsch technology and then upgraded to transportation fuels using usual refining methods. Alternatively, synthesis gas can be converted to liquid oxygenates which in turn can be converted to more conventional transportation fuels via catalysts such as certain zeolites.
Because synthesis gas processing requires high capital investment, with the syngas being produced in relatively energy intensive ways, such as by steam reforming where fuel is burned to supply heat for reforming, and represents an indirect route to the production of hydrocarbons, the search for alternate means of converting methane directly to higher hydrocarbons continues.
Oxidative coupling has been recognized as a promising approach to the problem of methane conversion although the mechanism of action is not, to date, completely understood. In such processes, methane is contacted with solid materials referred to by various terms including "catalyst", "promoters", "activators" or "contact materials", for example. Methane mixed with oxygen and catalyst is directly converted to ethane, ethylene, higher hydrocarbons and water. Carbon dioxide formation, which is highly favored thermodynamically, is an undesirable product associated with oxidative coupling as both oxygen and carbon are consumed without production of the desired higher value C.sub.2 + hydrocarbons. In addition, many methods for oxidative conversion have been carried out in the absence of an oxygen-containing gas, theoretically relying on oxygen being supplied by the catalyst.
Catalytic mixtures of yttrium-barium-copper oxides are highly active and 100% selective for producing CO.sub.2. Such catalysts which are highly selective for carbon dioxide formation are commonly referred to as "combustion catalysts". In order to obtain increased selectivity to hydrocarbon formation, Group IA metals, particularly lithium and sodium, have been added or otherwise used in many such catalytic mixtures. Under the conditions used for oxidative coupling, however, such mixtures typically realize migration and loss of the alkali metal. Thus, the need for highly active, C.sub.2 + hydrocarbon selective and stable oxidative coupling catalyst and improved processes employing the same continues.
Many patents describe processes for converting methane to heavier hydrocarbons in the presence of reducible metal oxide catalysts. Most of these patents require or imply the need for a separate stage to re-oxidize the catalyst. These include U.S. Pat. No. 4,444,984 which teaches a reducible oxide of tin as a catalyst; U.S. Pat. No. 4,495,374 disclosing the use of any reducible oxide promoted by an alkaline earth metal; 4,523,049 showing a reducible oxide catalyst promoted by an alkali or alkaline earth metal, and requiring the presence of oxygen during the oxidative coupling reaction. U.S. Pat. No. 4,656,155 specifies yttrium in a mixture requiring zirconium and alkali metal. U.S. Pat. No. 4,450,310 claims coupling promoted by alkaline earth oxides in the total absence of molecular oxygen. U.S. Pat. No. 4,482,644 teaches a barium-containing oxygen-deficient catalyst with a perovskite structure. European patent application No. 198,251 covers a process conducted in the presence of free oxygen using a three component contact material of: (a) an oxide of calcium, strontium or barium, and optionally a material selected from the group consisting of chloride ions, compounds containing chloride ions, tin and compounds containing tin; (b) a sodium or potassium-containing material, and a Group IIA metal or a compound containing a Group IIA metal, and optionally a material selected from the group consisting of chloride ions, compounds containing chloride ions, tin and compounds containing tin; (c) a Group IA metal compound, and optionally a material selected from the group consisting of chloride ions, compounds containing chloride ions, tin and compounds containing tin.
U.S. Pat. No. 3,885,020, although disclosing contact materials of the oxidative coupling type, is directed to a method of converting hydrocarbons to CO.sub.2 and water for pollution control. The combustion catalysts used consist of four components: (1) zirconium, tin or thorium; (2) an alkaline earth material; (3) a rare earth-type element such as scandium, lanthanum or cesium; and (4) a metal of the first transition series.
Baerns U.S. Pat. No. 4,608,449 relates to a methane conversion process using a suitable metal oxide catalyst, including tin oxide, on an oxide catalyst carrier carried out under temperatures of from 500.degree. C. to 900.degree. C. in the presence of oxygen at specified pressure.
Hicks U.S. Pat. No. 4,780,449 discloses a catalyst for the conversion of methane to hydrogen, and higher hydrocarbons comprising a non-reducible metal oxide of Be, Mg, Ca, Sr, Ba, Sc, Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu which may be used alone or with up to 50% by weight of one or more promoter oxides of Li, Na, K, Be, Mg, Ca, Sr, Ba, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sn, Pb, Sb, Bi, Cu, Ag and Au. Methane conversion is carried out at temperatures of from 500.degree. to 1000.degree. C.
Many analytical techniques have been developed to determine surface structure and/or composition of materials, e.g., solids. One such technique which has gained widespread application is Auger electron spectroscopy wherein electrons or photons, usually of 2 to 10 KeV, are used to excite core electrons of atoms of the solid with outer shell electrons "falling" into the electron vacancies created by the excited inner electrons. Such de-excitation of outer electrons into inner electron vacancies may lead to either an X-ray emission, or the energy may be given to another electron of the atom, commonly referred to as the Auger effect. Such Auger electrons have well-defined energies determined by the electron shells involved in the process, thus an Auger spectrum is characteristic of the atom and the environment about the atom. In Auger electron spectroscopy, the Auger electron is able to escape from the near surface region without appreciable energy loss; hence, the energy emission associated with such spectroscopy is primarily associated with surface or near surface atoms. Thus, one of the principal uses of Auger spectra is for the determination of the surface composition.
In general, quantum mechanics can be utilized to describe the discrete energy levels associated with the electrons of an atom. Thus, principal quantum numbers (n=1, 2, 3, 4 . . . ) correspond to electron shells (K, L, M, N . . . , respectively). Auger notation builds on atomic electron shell notation, with subscripted numbers referring to the shell angular momentum state involved. Thus, subscript "1" refers to the s shell angular momentum state, subscripts "2" and "3" refer to the pl/2 or p3/2 shell angular momentum states, and subscripts "4" and "5" refer to the d3/2 or d5/2 shell angular momentum states, respectively, for example.
Auger transition notations are referred to by three capital letters; e.g., KLL, KLM, LMM, MNN, etc. The first letter in the notation refers to the electron shell in which the initial electron vacancy associated with the excitation of core electrons during Auger emission occurs, e.g., MNN indicates that the initial vacancy was in the M electron shell. The second letter in the notation refers to the shell from which an electron comes to fill the initial vacancy; e.g., MNN indicates an N electron "drops" to fill the "hole" in the M electron shell. The third letter refers to the shell from which the Auger electron (as described above) is emitted or ejected; e.g., MNN indicates that the Auger electron is expelled from the N shell.
Thus, the Auger transition notation M.sub.4 N.sub.4,5 N.sub.4,5 means that the initial vacancy was in the M electron shell in the 3d3/2 state with the "hole" filled with a 4d electron from the N electron shell and a 4d electron from the N electron shell being expelled as the Auger electron. Conversely, the Auger transition notation M.sub.5 N.sub.4,5 N.sub.4,5 is for an initial vacancy in the 3d5/2 state of the M electron shell with the same two types of 4d electrons involved in the Auger decay. Thus, the splitting of the two Auger features roughly would equal the spin orbit splitting of the 3d states.